Common causes of weld failure and improvement measures?
Causes include: welding speed is too fast (>30m/min), groove angle is too small (<30°), misalignment is >1mm or welding deviation. Improvement measures: reduce welding speed to 15-20m/min, increase groove to 45°, and install laser centering system. ERW welded pipes need to check high-frequency impedance matching, and SAW welded pipes should adjust the wire extension length. Regular cleaning of roller oxides can also help improve fusion.
How to adjust the roller when the ovality of the welded pipe exceeds the tolerance?
First measure the elliptical direction. If the major axis is in the vertical direction, the horizontal roller needs to be adjusted upward; otherwise, it needs to be adjusted downward. The adjustment amount is based on the empirical value of 0.3mm ovality affected by every 0.1mm roll gap change. The finishing section needs to keep the deceleration rate of each run consistent (usually 10%-15%). For spiral welded pipes, the forming angle (usually 45°-60°) and the delivery line height should be checked. Laser diameter gauge feedback control can be used online for dynamic adjustment.
How to distinguish hot cracks from cold cracks in welding?
Hot cracks occur during solidification (above 1420℃), are distributed along the crystal, and have an oxidized color on the surface; cold cracks occur below 200℃, extend through the crystal, and have no oxidation characteristics. Hot cracks are mostly caused by sulfur-phosphorus segregation or low-melting point eutectics, and cold cracks are often caused by hydrogen embrittlement or residual stress. To prevent hot cracks, S≤0.010% must be controlled, and cold cracks require preheating (such as 150℃) and post-heating to remove hydrogen.
How to prevent zinc nodules on the surface of galvanized welded pipes?
Zinc nodules are caused by excessively high zinc liquid temperature (＞470℃) or too long zinc immersion time (＞60s). Optimization plan: Control the zinc liquid temperature at 450-460℃, and add 0.02% aluminum to reduce the iron-zinc reaction. The air knife pressure is adjusted to 1-2kPa, and the angle is 15° to the pipe for blowing. Thoroughly pickle (HCl concentration 15%-20%) and assist plating (ZnCl₂+NH₄Cl) before plating. For pipes with zinc nodules, grind with a grinding wheel and then spray zinc paint.
How to identify "pseudo-defect" signals in ultrasonic testing?
False signals may come from coupling agent bubbles (random distribution), oxide skin reflections (fixed position) or geometric echoes (regular appearance). True defect signal characteristics: The waveform changes continuously during dynamic scanning, and the wave height is stable during static probe detection. Use multi-angle probes (such as 45°+70°) for cross-verification, or combine eddy current detection to eliminate surface interference. Digital ultrasonic equipment can filter noise through DAC curves.
